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I have an HP SAN storage array that has 2 fans on the back. They run at full speed all of the time, and I'd like to slow them down based on the temperature of the hard drives in the array.

The fans don't have any good exterior markings (other then 12V), so in the name of science, I sacrificed one of the pair.

There are 6 wires:

  • M+
  • M-
  • PROG
  • TACH
  • ENB
  • IDENT

I get that M+ and M- are for power and that TACH is probably to measure the speed of the fan, but any idea what the other 3 are? And based on those connection names, any idea on the right way to control the fan speed?

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  • \$\begingroup\$ Once, for a high school project, I've built a temperature PID regulator based on a 8052 uC, and using thermistors for measuring temperature. It was nice, but sadly has never been mounted on my (RIP) PC. \$\endgroup\$ – clabacchio Feb 8 '12 at 8:03
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Could you post the picture of what's left of the fan, which you have dissected/sacrificed? Does it have a built in controller or motor driver?

If you have an oscilloscope - probe the lines. You can also disconnect one line at a time and see how the fan and the host will react. This could shed some light on the purpose of the lines.

Most fans have only 3 lines: motor+, ground, tach. Without knowing anything else, here's an educated guess about the 3 remaining lines:

PROG is for factory use. Perhaps, the fan has a built in memory (?) for calibration parameters (?).

ENB is a logic level on/off signal. Some times, it can be used for PWM speed control.

IDENT allows the host to identify the fan. On the fan, it could be connected to one of the following:
a resistor, which identifies the type of the fan
a nonvolatile memory
shorted to some other pin, which can tell the host whether the fan is connected at all or not

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    \$\begingroup\$ A lot of field-replaceable parts for enterprise equipment have small serial ROMs or Flash RAMs with FRU information (FRU order number, manufacture information, etc). It's weird getting a fan assembly with a ROM chip, though. \$\endgroup\$ – Alexios Feb 9 '12 at 23:22
  • \$\begingroup\$ Unless it's a very fancy HP san, I would expect IDENT to merely be a short to let the host know it has a fan, rather than having a fan which isn't turning (yet). ENB to turn the fan off an on, and PROG to provide the PWM. But post some pictures of the fans and the boards in the hubs of the fans, and the PCB near where the fan connects. \$\endgroup\$ – james Jan 15 at 12:33

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